At least with the Note 8, pretty much everyone got around the release date, may be max a day early from someone.
Earlier TMobile used to ship early but past couple of release Samsung has put a tight lid and controlling the release so that devices get delivered on launch day or a day early if you get lucky from some retailer.

I would say avoid best buy but this being a big release even they should have sufficient inventory to have launch day delivery or in store pick up. So just choose whoever you are familiar and comfortable with and you should get on launch day.

Trade in is "in store only", so won't work with pre-ordering online. It is unclear if you can go into BB on Friday and set up a pre-order with the discount and trade-in.
Typically that gift card cannot be used on the purchase of the item that it is granted for, it's intended for your next purchase.
The $100 discount is only for those that sign up with a payment plan, so paying it outright won't work. It also doesn't work for unlocked versions.

Honestly best bet would be to call best buy and ask their associates for more details, however the terms and conditions can be found here as well:

Can I preorder a S9 from Best Buy on March 2 (saving $100) and have it delivered to the store for pickup? I can do this either online or go into the store.

Then when the S9 comes in, go to the store to pick it up and trade in my old S6 for a $150 BB gift card.

Then use the gift card towards the cost of the new S9 thereby reducing the total cost.

I plan to pay upfront the whole cost of the new phone, no payment plan.

Will it work this way? Or am I overlooking something?

Which carrier are you going to buy it for?

Currently, only Sprint and Unlocked are live with prices. And only Sprint shows a $100 discount. That's automatic. BUT that's on the installment plan. If you buy it full-retail, there is no $100 savings.

For the trade-in, you'll need to do this on an in-store transaction, not an online pick-up/pre-order. And you wouldn't get the gift card until you've purchased the S9. So no, you won't be using that gift card to pay for the S9.

My feeling is you can pre-order online and then when picking it up IN STORE you can trade in for the additional trade-in value. The wording on their site says "Friday Only: Save $100 on the Samsung Galaxy S9 or S9+ with pre-order and qualified activation. Plus, save up to $350 with qualifying trade-in."

I'm taking the word PLUS to mean you can take advantage of both offers. This is what I did when going from the S7 edge to the S8+ (pre-ordered online, traded in for S7E value when picking up), though that's no guarantee this will be the same. But the wording of the promotion ("plus") sounds pretty clear to me.

At least with the Note 8, pretty much everyone got around the release date, may be max a day early from someone.
Earlier TMobile used to ship early but past couple of release Samsung has put a tight lid and controlling the release so that devices get delivered on launch day or a day early if you get lucky from some retailer.

I would say avoid best buy but this being a big release even they should have sufficient inventory to have launch day delivery or in store pick up. So just choose whoever you are familiar and comfortable with and you should get on launch day.

Best buy actually sent out Verizon note 8 devices a week early which was a nice surprise

At least with the Note 8, pretty much everyone got around the release date, may be max a day early from someone.
Earlier TMobile used to ship early but past couple of release Samsung has put a tight lid and controlling the release so that devices get delivered on launch day or a day early if you get lucky from some retailer.

I would say avoid best buy but this being a big release even they should have sufficient inventory to have launch day delivery or in store pick up. So just choose whoever you are familiar and comfortable with and you should get on launch day.

I had my Note 8 over a week before the official release date. I ordered through Best Buy.

I spoke with a Best Buy mobile store today and asked them if I trade in the S7 active I have laying around if I can use that gift card towards the down payment of the S9 and he said absolutely I could do that I'm actually going to trade my S7 active in on Friday since I really have no use for it right now.